This invention relates to flame-retardant thermoplastic compositions having improved dielectric properties and a process for the preparation thereof. More particularly, it pertains to compositions comprising a high molecular weight linear polyester, a flame-retardant, novaculite, and, optionally, a reinforcing agent and a process for improving the arc track resistance of reinforced, flame-retardant linear high molecular weight polyester compositions.
High molecular weight linear polyesters and polymers of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
With the development of molecular weight control, use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection modlable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
One useful family of such compositions comprises those which are reinforced, e.g., with from about 10 to about 40% of filamentous glass, based on the weight of glass and polyester components, and rendered flame-retardant by the incorporation of a flame-retarding amount of a flame-retarding component.
Another useful family of such compositions comprises those containing poly(1,4-butylene terephthalate), a polycarbonate resin, a flame-retardant and, optionally, a reinforcing agent.
Experience has show, however, that, typically, articles molded from these afore-described polyester compositions are characterized with poor arc resistance properties.
For example, the high voltage track rate, as measured according to Underwriter's Laboratory Bulletin 494, of a 30% glass-reinforced, non-flame-retardant poly(1,4-butylene terephthalate) composition is 0.50 to 0.87 inches per minute. The addition of a flame-retardant to this same 30% glass reinforced poly(1,4-butylene terephthalate) composition results in a track rate of 8.2 to 11.6 inches per minute. The higher rate indicates less desirable arc track resistance.
It has now been surprisingly discovered that the incorporation of a particulate silica filler, i.e., novaculite, improves the high voltage arc track resistance of reinforced or unreinforced flame-retarded polyester resins while not adversely affecting the flame-retardancy or the other excellent and desirable physical properties of the composition.